The size and diversity of the immune repertoire found in naive peripheral lymphocytes must be maintained at a sufficient capacity to mount effective immune responses to new infectious agents. This repertoire is constantly diminished by pathogen challenge. It is currently postulated the naive B cell pool is maintained through interactions with ligands in the environment that promote lymphocyte survival. We have recently demonstrated that adoptively transferred B lymphocytes actively replicate in B cell deficient hosts and maintain their naive phenotype. This homeostatic proliferation (HP) of mature B cells requires Bruton's tyrosine kinase is T cell and antigen independent, inhibited at high B cell densities and displays signaling requirements distinct for those for antigen/mitogen induced activation or for peripheral B cell survival. We propose that homeostatic B cell proliferation is ongoing, not just confined to lymphopenic settings and constitutes an antigen-independent mechanism for maintaining and expanding the immune repertoire of the naive B cell pool. Understanding the regulation of this homeostatic mechanism could provide important new therapies for effecting the reconstitution of immune systems debilitated by chemotherapy, virus infection or aging. We propose to elucidate this homeostatic mechanism using the following rationale and experimental approaches. Extensive B cell traffic to the spleen makes it an ideal site for monitoring peripheral B cell density and providing the cells and ligands necessary to initiate HP. Aim 1 will determine if all B cell subpopulations can undergo HP, the accessory cells and microarchitectural elements in the spleen required for HP and if sites outside the spleen can support HP. Basal signals from the BCR are necessary for B cell survival and intensification or costimulation of these signals may induce HP. In Aim 2 we will determine if the BCR is necessary for HP, if HP can be initiated in any B cell or is confined to a cohort with a restricted BCR repertoire and the signaling pathways required to induce HP. B cell survival is also dependent on stimulation by the TNF-related ligand BLyS. Aim 3 will use adenovirus-mediated gene transfer to determine if costimulation with BLyS is required for or enhances HP, which of the BLyS receptors is necessary for this stimulation and the effect, on mature B cell homeostasis, of selectively inhibiting the expression of BLyS receptors TACI or BR3.